I know little about Judaism and it's a first time question, so please forgive any mistakes.

According to this question non-Jews are forbidden from keeping Shabbat. What is the reasoning behind this? I would have expected keeping Shabbat to be a God-honouring thing, especially in the cited case of a person considering conversion.

3 Answers
3

This article brings several reasons why gentiles are forbidden from observing Shabbos:

1. The Rambam in codifiying this law (Melachim uMilchamot 10:9) explains that the issue is in gentiles innovating their own laws:

Similarly, a gentile who rests, even on a weekday, observing that day
as a Sabbath, is obligated to die. Needless to say, he is obligated
for that punishment if he creates a festival for himself. The general principle governing these matters is: They are not to be allowed to originate a new religion or create mitzvot for themselves based on their own decisions. They may either become righteous converts and accept all the mitzvot or retain their statutes without adding or detracting from them.

2. The Maharsha (Sanhedrin 58b) compares Shabbos to the bride of the Jewish people, and compares gentiles observing it to adultery:

The Maharsha explains that the Sabbath is, metaphorically, a bride.
Indeed, the Talmud refers to the Sabbath as a bride and the Shabbos is
greeted in the same way that a bride is greeted. This imagery was
immortalized by Rabbi Shlomo Alkabetz (a 16th century poet and
Kabbalist from Tzfas) in his classical liturgical song, Lecha Dodi,
which is sung just before the reception of the Holy Shabbos. Rabbi
Avraham Sperling writes, based on this allegorical comparison, that
“Shabbos” is the female companion to the Israelite nation. The Jewish
Nation is “married” to Shabbos. Therefore, when a non-Jew follows the
rules of Shabbos, it is as if he is committing “adultery” with the
married bride “Shabbos”, and so he is liable for the death penalty.

3. He suggest (based on Rashi's explanation) that the resting of Shabbos is an exclusive gift to the Jews:

When Adam sinned by
eating the Forbidden Fruit, he was punished by all future males having
to work, “with the sweat of his brow.” This implies that HaShem
expects man to work continuously without rest. However, two thousand
four hundred and forty-eight years later, HaShem granted the Jews a
special present, namely, the Holy Sabbath, with which they can
rest, in contrast to the remainder of society. Therefore, world
society is not allowed to rest because of the curse of Adam, while the
Jews are allowed to rest because the Torah specifically calls for a
Mitzvah of Shabbos, which was a unique gift granted to the Jewish
Nation . . Rabbi Meir ben Todros HaLevi Abulafia (1170-1244) explains that the gift of Shabbos was given specifically to the Jews, and therefore if a Noahide keeps Shabbos, he is actually stealing from the Jews and is therefore liable for the prohibition of stealing (which is one of the Seven Noachide Laws).

Although I haven't had time to look at the sources mentioned, there appears to be an opinion by Rashi (cited here) that people dwelling the land of Israel who are obligated to keep the 7 Noahide Laws, must also keep the Sabbath, and apparently Rabbi Chaim Kanievsky (cited in that forum thread) wrote that gentiles in the diaspora today have the same status as gentiles living in the Land of Israel in ancient times, including keeping the Sabbath (referring back to the aforementioned opinion of Rashi).

I have NOT heard of either of these opinions quite as stated above. I'd have to look at the sources mentioned, and I don't have time right now. There is, however, a verse about the Sabbath that includes the language "and the stranger within your gates" (Zachor). It does sound from here that non-Jews in Israel may not perform any work on the Sabbath (and I've heard this as a potential problem in modern-day Israel).

But, in a nutshell, it is widely held, at least outside of Israel, that non-Jews are specifically not allowed to keep the Sabbath because the Sabbath is called an eternal "sign" (some render this as "covenant") between G-d and Israel. (See here, for a short discussion on the topic.)

I think the drasha is from just לא ישבותו not the יום ולילה.
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Double AA♦Oct 7 '12 at 3:35

@DoubleAA: The Talmud quotes the verse starting from "and day...' - and from Rashi it seems that that is part of the limud. Otherwise we would think it was referring the 6 seasons mentioned earlier in the verse.
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MenachemOct 7 '12 at 3:40

This explains the source of the law, not the reason it is forbidden.
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MichoelOct 7 '12 at 4:55